Tool made of cubic boron nitride sintered body

ABSTRACT

A tool includes a cubic boron nitride sintered body at least at a tool working point. The cubic boron nitride sintered body contains cubic boron nitride, a heat insulating phase, and a binder phase. Cubic boron nitride is contained in the cubic boron nitride sintered body by not less than 60 volume % and not more than 99 volume %, and the heat insulating phase includes one or more types of first compound composed of one or more types of element selected from the group consisting of Al, Si, Ti, and Zr and one or more types of element selected from the group consisting of N, C, O, and B. The first compound is contained in the cubic boron nitride sintered body by not less than 1 mass % and not more than 20 mass % and it has an average particle size smaller than 100 nm.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a tool made of a cubic boron nitridesintered body and particularly to a tool made of a cubic boron nitridesintered body excellent in wear resistance and chipping resistance.

2. Description of the Background Art

In cutting a material, a cutting tool and a cutting method suitable fora work material are selected. In order to achieve a long life duringcutting, it is important how a temperature at a cutting edge duringcutting can be suppressed, and a tool material having excellent thermalconductivity is valued. In general, also during cutting using a toolmade of an ultra-high pressure sintered body such as a diamond sinteredbody and a cubic boron nitride (which may also be denoted as “cBN”)sintered body excellent in thermal conductivity, diffusion into a workmaterial or chemical wear such as oxidation develops due to increase intemperature at the cutting edge under such a high-efficiency conditionas a high-speed condition, a large cutting-depth condition, or ahigh-feed condition. As measures for suppressing such wear of a tool,change to a low-speed condition, suppression of resistance duringcutting by decreasing a wedge angle at the cutting edge of the tool,cooling of a cutting point by injecting a coolant toward the cuttingpoint, or the like has been carried out.

For example, as measures for achieving a further longer life duringcutting of a difficult-to-machine material, Japanese Patent Laying-OpenNo. 2009-045715 discloses an invention for suppressing increase intemperature at a cutting edge due to heat generated by cutting, bycarrying out working while the cutting edge of a cutting tool, in whichan ultra-high pressure sintered body material having such a high heatradiation property as thermal conductivity not lower than 100 W/m·K isapplied to a portion of the cutting edge at least involved with cutting,is cooled with a high-pressure coolant.

Meanwhile, for example, during cutting of a brittle difficult-to-machinematerial such as glass, ceramics, cemented carbide, or an iron-basedsintered alloy difficult-to-machine material, it has been proposed toachieve a good worked surface by softening a work material or varying amechanism of generation of chips from a brittleness mode to a ductilitymode by carrying out cutting under a high-speed condition or byincreasing a temperature at a point of cutting of the work material withlaser assistance.

In principle, however, the cutting edge of the tool is exposed to a hightemperature and also to rapid cooling, and hence a cutting tool tends todegrade and chipping or sudden chipping thereof is likely. In addition,in a machine tool as well, such problems as restriction on the number ofrevolutions of a main shaft or requirement for installation of anexpensive laser apparatus arise.

A cBN sintered body mainly refers to a body obtained by bonding cBNparticles to one another with a binder mainly composed of TiN, TiC, Co,and Al. The cBN particles are a material having hardness and thermalconductivity next to diamond and being superior in toughness to aceramics material. Therefore, a cBN sintered body having such a high cBNcontent that it contains cBN particles by 80 volume % or more isexcellent in such characteristics as resistance to plastic deformation,chipping resistance, and the like.

A tool made of a cBN sintered body, which includes the cBN sintered bodyhigh in cBN content and having such characteristics, is excellent inthat it is better in chemical stability, lower in affinity with iron,longer in life, and higher in efficiency in working because of its highhardness as a material, than a tool material such as a conventionalsuperhard tool and the like, and it is highly evaluated. Such a toolmade of a cBN sintered body of high performance has replaced aconventionally used tool in such applications as cutting of Ni-based andiron-based high-hardness difficult-to-machine materials, applications ofplastic working of a punching tool for cold forging, and the like.

Here, cutting refers to machining of an article having desired dimensionand shape while a work material is locally sheared and crushed and chipsare generated, whereas plastic working refers to application of force toa workpiece to deform the same and formation of the workpiece into aproduct having prescribed shape and dimension. It is noted that plasticworking is different from cutting in that no chips are generated.

Since the tool made of the cBN sintered body has excellentcharacteristics as described above, it is advantageous in that suddenchipping is less likely in any application of cutting and plasticworking and it is extremely suitably employed.

For example, Japanese Patent Laying-Open No. 07-291732 and JapanesePatent Laying-Open No. 10-158065 each disclose as a conventional toolmade of a cBN sintered body, with such a metal as Al, oxygen, and thelike contained in a cBN sintered body being regarded as an impurity, atechnique for improving hardness and toughness of a cBN sintered body byminimizing introduction of such an impurity and increasing a ratio ofcBN particles to be mixed.

In addition, a tool made of a cBN sintered body has been considered andcommonly believed to be high in performance if it has high hardness andhigh toughness as well as high thermal conductivity. In accordance withthis common belief, Japanese Patent Laying-Open No. 2005-187260 andWO2005/066381 each have proposed, by making use of high thermalconductivity of high-purity cBN particles, a tool made of a cBN sinteredbody which achieves improved hardness and toughness as well as improvedthermal conductivity by including a cBN sintered body containinghigh-purity cBN particles at high concentration. Chipping of such a toolmade of a cBN sintered body is less likely even in a case of plasticworking of a material of low ductility, in particular in a case ofcutting of an iron-based sintered alloy, and the tool is excellent alsoin wear resistance, whereby the tool is suitably used.

SUMMARY OF THE INVENTION

In a case where a tool made of a cBN sintered body high in cBN contentis applied to cutting of a recent difficult-to-machine material havinglow ductility characteristics, however, since the cBN sintered body hashigh thermal conductivity, friction heat generated in a worked portionduring cutting diffuses into the cBN sintered body. Consequently,cutting cannot proceed while a high temperature of thedifficult-to-machine material is maintained and hence cutting efficiencybecomes significantly poor.

Namely, a sintered body high in cBN content in which a cBN sintered bodycomponent occupies 80 volume % or more is excellent in chippingresistance. At the same time, however, such a sintered body has highthermal conductivity exceeding 70 W/m·K, and hence friction heatgenerated through working escapes from the cBN sintered body. Therefore,since the work material does not soften due to insufficient conductionof heat generated during working to the work material, load is imposedon the tool and even the tool made of the cBN sintered body high inchipping resistance is chipped.

In particular during cutting of an iron-based sintered alloy, because ofits low ductility, in a cutting environment where a temperature of awork material is insufficient, shear does not smoothly proceed, pits arecreated in a worked surface, and surface roughness may become poor. Whena cutting speed is increased in order to improve surface roughness, thatis, a temperature of the work material is raised, wear rapidly developsand a satisfactory tool life cannot be obtained. Alternatively, in acase of shear-cutting of an ultra-heat-resistant alloy represented by anNi-based alloy excellent in hardness at a high temperature or also in acase where corresponding heat generated by working flows into a workmaterial, a work material is less likely to soften because of itscharacteristics of excellent hardness at a high temperature and hencethe cBN sintered body is likely to be chipped.

It is estimated that a main factor for such chipping caused in a cBNsintered body would be a mechanism of mechanical damage such as crush ofcBN particles themselves due to insufficient strength or conspiredfalling-off of cBN particles due to insufficient binding force among thecBN particles.

A tool made of a cBN sintered body is required to be further higher inperformance also in plastic working. Namely, in plastic working, withhigher performance of a workpiece, working with cold forging in a caseof plastic working of a difficult-to-work material having suchcharacteristics as high hardness and low ductility is likely to causesuch defects as cracks or fractures in the workpiece. Thus, only afterhardness of the workpiece is lowered and ductility thereof is enhancedby heating the workpiece to a temperature not lower than 400° C. and nothigher than 1000° C. as in warm forging, hot forging, and the like, theworkpiece should be subjected to plastic working. In a case of plasticworking with warm forging, hot forging, or the like, however, atemperature of a worked portion becomes higher by friction heatgenerated at the worked portion than in a case of working with coldforging, load is imposed on the tool by the influence from the hightemperature, and consequently a life of the tool has extremely beenshort.

In addition, plastic working of a steel material containing carbon in anamount not less than 0.5 mass % will generate a brittle layer having amartensite structure or retained austenite, because a cBN sintered bodyhas high thermal conductivity, heat generated by working rapidly flowsout to a tool made of the cBN sintered body, and a workpiece is rapidlycooled. Material strength and fatigue strength of the workpiece thusalso tend to degrade.

If a cBN content is less than 80 volume % in order to prevent rapidcooling of a workpiece, thermal conductivity becomes relatively low andheat generated by working is less likely to flow out to the tool made ofthe cBN sintered body and hence rapid cooling of the workpiece can besuppressed. On the other hand, a binder phase poorer in strength andtoughness than cBN particles becomes relatively dominant, and hence thetool made of the cBN sintered body may be chipped in an early stage.

With such an approach to increase and decrease a content of cBNparticles, improvement in hardness of a tool and lowering in thermalconductivity of the tool have trade-off relation, and it has beendifficult to satisfy both of them.

The present invention was made in view of the circumstances as above,and an object thereof is to provide a tool made of a cubic boron nitridesintered body that achieves both of lowering in thermal conductivity ofa cubic boron nitride sintered body and improvement in hardness of thetool.

The present inventors have clarified characteristics required inapplications of cutting and plastic working described above and havedeveloped materials. Consequently, the present inventors have foundthat, by containing a cBN component by not less than 60 volume % and notmore than 99 volume % at the time of fabrication of a cBN sintered bodyand by adding an intermetallic compound in a form of fine particles, ofAl, Si, Ti, Zr, or the like, to a component of a binder phase, acompound composed of one or more types of element selected from thegroup consisting of Al, Si, Ti, and Zr and one or more types of elementselected from the group consisting of N, C, O, and B, which has anaverage particle size smaller than 100 nm, can be a heat insulatingphase for lowering thermal conductivity.

In addition, the present inventors have found that, since each componentof an ultra-fine compound above is poor in sinterability, unsinteredregions scatter in a part of a cBN sintered body during ultra-highpressure sintering and consequently thermal conductivity of the cBNsintered body can be lowered. By conducting further dedicated studiesbased on such findings, the present inventors have finally completed thetool made of the cBN sintered body according to the present invention.

Namely, the present invention is directed to a tool made of a cubicboron nitride sintered body which includes a cubic boron nitridesintered body at least at a tool working point, the cubic boron nitridesintered body contains cubic boron nitride, a heat insulating phase, anda binder phase, cubic boron nitride is contained in the cubic boronnitride sintered body by not less than 60 volume % and not more than 99volume %, the heat insulating phase contains one or more types of firstcompound composed of one or more types of element selected from thegroup consisting of Al, Si, Ti, and Zr and one or more types of elementselected from the group consisting of N, C, O, and B, the first compoundis contained in the cubic boron nitride sintered body by not less than 1mass % and not more than 20 mass % and has an average particle sizesmaller than 100 nm, and the cubic boron nitride sintered body hasthermal conductivity not higher than 70 W/m·K.

The first compound preferably has an average particle size smaller than50 nm. In addition, preferably, the heat insulating phase contains asits part, an unsintered region by not less than 0.01 volume % and notmore than 3 volume % with respect to the cubic boron nitride sinteredbody.

Further preferably, the first compound is a compound in which a solidsolution of any one or both of oxygen and boron is present by not lessthan 0.1 mass % and not more than 10 mass % with respect to a nitride, acarbide, and a carbonitride of one or more types of element selectedfrom the group consisting of Al, Si, Ti, and Zr.

Preferably, the heat insulating phase contains one or more types ofsecond compound composed of W and/or Re and one or more types of elementselected from the group consisting of N, C, O, and B, in addition to thefirst compound, and the second compound is contained in the cubic boronnitride sintered body by not less than 0.1 mass % and not more than 2mass %.

Preferably, cubic boron nitride is contained in the cubic boron nitridesintered body by not less than 75 volume % and not more than 92 volume%, and more preferably cubic boron nitride is contained in the cubicboron nitride sintered body by not less than 80 volume % and not morethan 87 volume %.

Preferably, cubic boron nitride is composed of cubic boron nitrideparticles having an average particle size not greater than 1 andpreferably the cubic boron nitride sintered body has thermalconductivity not higher than 60 W/m·K.

Preferably, the tool working point has surface roughness Rz not lessthan 1 μm and not more than 20 μm, and preferably the cubic boronnitride sintered body has a minimum thickness not smaller than 2 mm atthe tool working point.

Preferably, the cubic boron nitride sintered body and a tool shankportion are fixed to each other with a vibration-isolatingheat-resistant plate being interposed, and the vibration-isolatingheat-resistant plate is made of an oxide, has thermal conductivity nothigher than 40 W/m·K, and has a thickness not smaller than 0.3 mm.

Preferably, the cubic boron nitride sintered body and the tool shankportion are fixed to each other by screwing and/or self-gripping.

By having the features above, a tool made of a cubic boron nitridesintered body according to the present invention has an effect toachieve both of lowering in thermal conductivity and improvement inhardness of the tool made of the cubic boron nitride sintered body, andhence it is excellent in wear resistance and chipping resistance.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Each feature of a tool made of a cubic boron nitride sintered bodyaccording to the present invention will be described further below.

<Tool Made of Cubic Boron Nitride Sintered Body>

A tool made of a cubic boron nitride sintered body according to thepresent invention has a construction including a cBN sintered body atleast at a tool working point. Specifically, the tool made of the cBNsintered body according to the present invention preferably has such aconstruction that the cBN sintered body is fixed to a tool shank portionwith a vibration-isolating heat-resistant plate being interposed. Thetool made of the cBN sintered body according to the present inventionhaving such a construction can particularly effectively be used inmachining of an iron-based sintered alloy, a difficult-to-machine castiron, or the like, and in addition it can suitably be used also invarious types of working of general metals other than the former. Here,the “tool working point” refers to a portion of a surface of the toolmade of the cBN sintered body which comes in contact with a workpiece.It is noted that the tool shank portion and the vibration-isolatingheat-resistant plate will be described later.

In using the tool made of the cBN sintered body according to the presentinvention in an application of cutting, for example, it can extremelyusefully be employed as a drill, an end mill, a coated cutting insertfor milling or turning, a metal saw, a gear cutting tool, a reamer, atap, or a tip for crankshaft pin milling, a cutter for cutting a glasssubstrate, an optical fiber cutter, and the like.

On the other hand, in using the tool made of the cBN sintered bodyaccording to the present invention in an application of plastic working,it can extremely usefully be employed as a die for punch pressing, a diefor dicing, a tool for friction welding, a tool for friction stir joint,or the like. Then, in plastic working, for example, the tool made of thecBN sintered body is used for forming, for example, an engine component,an HDD (hard disk drive), an HDD head, a capstan, a wafer chuck, asemiconductor transportation arm, components in an automobile drivesystem, a zoom lens sealing ring for a camera, or the like.

<Cubic Boron Nitride Sintered Body>

The cBN sintered body according to the present invention preferablycontains cubic boron nitride, a heat insulating phase, and a binderphase. As the cBN sintered body thus contains the heat insulating phase,thermal conductivity of the cBN sintered body can be lowered and thermalconductivity thereof can be not higher than 70 W/m·K. When the tool madeof the cBN sintered body which includes the cBN sintered body havingsuch low thermal conductivity is used for cutting or plastic working,friction heat and shear heat generated during working conducts to aworkpiece rather than to the tool made of the cBN sintered body. Theworkpiece is thus likely to soften, load imposed on the cutting edge ofthe tool made of the cBN sintered body can be lowered, and hence wearand chipping of the tool made of the cBN sintered body can be lesslikely. Thermal conductivity of the cBN sintered body not higher than 60W/m·K can promote softening of a workpiece, and wear and chipping of thetool made of the cBN sintered body are less likely, which is furtherpreferred. Further preferably, thermal conductivity of the cBN sinteredbody is not higher than 50 W/m·K.

By thus lowering thermal conductivity of the cBN sintered body, cuttingperformance can be improved and surface roughness of a worked surface ofa work material can also be improved. The reason is estimated asfollows. As the work material softens, shear of the work material at atool working point can smoothly proceed, and hence generation of pitsand the like is less likely in a worked surface and a good workedsurface can be obtained.

Here, a minimum thickness of the cBN sintered body at the tool workingpoint is preferably not smaller than 2 mm and more preferably notsmaller than 3 mm. In a case where the minimum thickness of the cBNsintered body at the tool working point is smaller than 2 mm, when awidth of wear exceeds 2 mm, working is carried out by the tool shankportion and then a life is extremely shortened. Here, the “minimumthickness” refers to a thickness of a thinnest portion of the cBNsintered body.

The tool working point preferably has surface roughness Rz not less than1 μm and not more than 20 μm. When Rz is less than 1 μm, friction heatis less likely to be generated at the tool working point, a temperatureof a work material does not sufficiently increase at the working point,and hence chipping may be more likely. On the other hand, when Rzexceeds 20 μm, a component of a workpiece tends to adhere to the cuttingedge during working and surface roughness of the workpiece may degrade.From a point of view of improvement in tool life and better surfaceroughness of the workpiece, Rz is more preferably not less than 1.5 μmand not more than 10 μm and further preferably not less than 2 μm andnot more than 5 μm. It is noted that, in the present invention, surfaceroughness Rz refers to 10-point average roughness defined under JISB0601 and a measurement value obtained with the use of a surfaceroughness measuring instrument (SURFCOM 2800E (manufactured by TokyoSeimitsu Co., Ltd.)) is adopted.

<Cubic Boron Nitride>

The present invention is characterized in that cubic boron nitride iscontained in the cBN sintered body by not less than 60 volume % and notmore than 99 volume %. Here, when cBN in the cBN sintered body is lessthan 60 volume %, wear resistance is insufficient. When cBN exceeds 99volume %, the binder phase becomes relatively less and bonding strengthlowers. In consideration of balance between wear resistance and bondingstrength, a content of cBN is more preferably not less than 75 volume %and not more than 92 volume % and further preferably not less than 80volume % and not more than 87 volume %.

Here, the cBN sintered body is preferably sintered, with cBN particles,source material powders of a first compound forming the heat insulatingphase, and source material powders forming the binder phase beingincluded. From a point of view of a strengthened effect of improvementin material strength and lowering in thermal conductivity, the cBNparticles more preferably have a small average particle size and the cBNparticles preferably have an average particle size not greater than 1μm. In addition, from a point of view of not impairing toughness of thecBN sintered body, the cBN particles preferably have an average particlesize not smaller than 0.1 μm. From a point of view of balance amongmaterial strength, thermal conductivity, and toughness, the cBNparticles further preferably have an average particle size not smallerthan 0.2 μm and not greater than 0.5 μm. An average particle size of cBNparticles is preferably measured, for example, with a method of lappinga cBN sintered body to a mirror surface, magnifying the cBN sinteredbody with an electron microscope, measuring a particle size of the firstcompound in the heat insulating phase at a plurality of sites, andcalculating an average value.

<Binder Phase>

In the present invention, the binder phase contained in the cBN sinteredbody exhibits a function to bond the cBN particles to one another andany binder phase having conventionally known composition which has beenknown as a binder phase of a cBN sintered body can be adopted. Forcomposition used for the binder phase, a compound composed of at leastone type of element selected from the group consisting of Ti, W, Co, Zr,and Cr, one or more types of element selected from the group consistingof N, C, O, and B, and Al is preferred, and a compound of Al and atleast one type of carbide, boride, carbonitride, oxide, and solidsolution of at least one type of element selected from the groupconsisting of Ti, W, Co, Zr, and Cr is further preferred. Thus, inmachining of an iron-based sintered alloy, cast iron, or the like,particularly good wear resistance can be obtained. In particular, as Cois employed as a main component as a material to be used for the binderphase, chipping resistance of the tool made of the cBN sintered body canbe improved.

<Heat Insulating Phase>

In the present invention, as the heat insulating phase scatters in thecBN sintered body, it can lower thermal conductivity of the cBN sinteredbody. Therefore, heat generated during working is less likely to conductto the tool made of the cBN sintered body but conduction thereof to aworkpiece is promoted. Such a heat insulating phase is composed of amaterial poor in sinterability, and specifically the heat insulatingphase contains one or more types of first compound composed of one ormore types of element selected from the group consisting of Al, Si, Ti,and Zr and one or more types of element selected from the groupconsisting of N, C, O, and B, the first compound is contained in the cBNsintered body by not less than 1 mass % and not more than 20 mass %, andit has an average particle size smaller than 100 nm. When the firstcompound is less than 1 mass %, an effect of lower thermal conductivityof the cubic boron nitride sintered body cannot sufficiently be obtainedand conduction of heat to a workpiece is not promoted. On the otherhand, when the first compound exceeds 20 mass %, sintering isinsufficient and hardness of the cubic boron nitride sintered body islowered. Meanwhile, when the first compound has an average particle sizenot smaller than 100 nm, thermal conductivity of the cubic boron nitridesintered body exceeds 70 W/m·K, and the effect of the present inventioncannot be obtained. From a point of view of lowering in thermalconductivity of the cubic boron nitride sintered body, the firstcompound preferably has an average particle size smaller than 50 nm.

Such a heat insulating phase preferably contains as an unsinteredregion, the first compound in the cBN sintered body. The “unsinteredregion” in the present invention refers to a region around a grainboundary and an interface where a reactant in a form of particles orfine layers caused by sintering, that is formed at an interface betweenthe heat insulating phase and the cBN particles, does not exist, and toa region including particles in contact with that region. Such anunsintered region is included preferably by not less than 0.01 volume %and not more than 3 volume % with respect to the cBN sintered body. Whenthe unsintered region is less than 0.01 volume %, an effect as the heatinsulating phase cannot sufficiently be obtained, which is notpreferred. When the unsintered region exceeds 3 volume %, strength ofthe cBN sintered body lowers, which is not preferred.

Though a detailed mechanism for the heat insulating phase to include anunsintered region has not been clarified, it is possibly estimated asfollows. When cBN particles, source material powders of the firstcompound, and source material powders forming the binder phase are mixedand sintered at an ultra-high pressure, an average particle size of thesource material powders of the first compound is smaller than an averageparticle size of the source material powders forming the binder phase.Therefore, a pressure on the source material powders of the firstcompound is not sufficiently transmitted and the unsintered region in aform of fine layers is formed at the interface between the heatinsulating phase, and the binder phase and the cBN particles around thesame.

In the present invention, an unsintered region can be confirmed as aregion occupied by particles in contact with the grain boundary where aregion in which both elements of the heat insulating phase and the cBNcomponent are simultaneously detected does not essentially exist, byusing a transmission electron microscope (TEM) attached with an energydispersive X-ray spectroscopy (EDX) apparatus, an Auger electronmicroscope, or a secondary electron microscope. In addition, volume % ofan unsintered region occupied in the cBN sintered body is calculatedbased on a ratio of an area occupied by the unsintered region to an areaof a cut surface when the cBN sintered body is cut across one plane.

The first compound above is preferably a compound in which a solidsolution of any one or both of oxygen and boron is present preferably bynot less than 0.1 mass % and not more than 10 mass %, more preferably bynot less than 0.2 mass % and not more than 7 mass %, and furtherpreferably by not less than 1 mass % and not more than 3 mass % withrespect to a nitride, a carbide, and a carbonitride of one or more typesof element selected from the group consisting of Al, Si, Ti, and Zr. Bycontaining oxygen and boron at such a ratio, an unsintered region havingan effect as the heat insulating phase is likely to be formed in the cBNsintered body, and hence a heat insulating property of the tool made ofthe cBN sintered body can be enhanced without impairing chippingresistance. In particular in a case where the first compound containsboron, a reactant in a form of particles or fine layers caused bysintering, that is formed at an interface between the heat insulatingphase and the cBN particles, refers to a region around a grain boundaryand an interface where boron is detected at concentration higher thanthe first compound.

Preferably, the cBN sintered body according to the present inventioncontains one or more types of second compound composed of W and/or Reand one or more types of element selected from the group consisting ofN, C, O, and B, in addition to the component of the first compoundabove, and the second compound is contained in the cBN sintered body bynot less than 0.1 mass % and not more than 2 mass %. Here, the secondcompound is a compound discontinuously arranged in the structure of thecBN sintered body. For example, ammonium paratungstate(5(NH₄)₂O.12WO₃₀.5H₂O) can be exemplified as a source materialcontaining W, and ammonium perrhenate (NH₄ReO₄) or the like can beexemplified as a material containing Re.

By mixing source material powders of the second compound (that is, forexample, powders composed of 5(NH₄)₂O.12WO₃.5H₂O or powders composed ofNH₄ReO₄), the source material powders forming the binder phase, and thecBN particles in addition to the source material powders of the firstcompound above and then subjecting the mixture to ultra-high pressuresintering, NH₄ and/or H₂O contained in the source material powders ofthe second compound function(s) as a catalyst in such ultra-highpressure sintering. Then, the function of this catalyst can bring aboutdirect bond among the cBN particles and hence strength of the cBNsintered body can be enhanced.

Further, by sintering the cBN particles together with the sourcematerial powders of such a second compound at an ultra-high pressure, W,Re, or an alloy of W and Re, and an oxide thereof excellent in hardnessat high temperature and toughness are discontinuously arranged in thestructure of the cBN sintered body, so that thermal conductivity of thecBN sintered body can consequently be lowered. Therefore, as such asecond compound is contained in the cBN sintered body, chippingresistance can be improved without lowering in wear resistance and heatresistance of the tool made of the cBN sintered body.

<Tool Shank Portion>

In the present invention, as the tool shank portion to which the cBNsintered body is fixed, any conventionally known tool shank portionwhich has been known as a tool shank portion of this type can beadopted, and it is not particularly limited. For example, a tool shankportion made of cemented carbide or stainless steel can suitably be usedas such a tool shank portion.

Here, the cBN sintered body above and the tool shank portion arepreferably fixed to each other by screwing and/or self-gripping. Byfixing the cBN sintered body with such a method, when the tool made ofthe cBN sintered body has worn and its function has been impaired, theworn cBN sintered body alone can be replaced. Thus, the tool shankportion can repeatedly be used without replacing the same.

<Vibration-Isolating Heat-Resistant Plate>

In the present invention, a vibration-isolating heat-resistant plate ispreferably interposed at a portion where the cBN sintered body and thetool shank portion are fixed to each other. By interposing thevibration-isolating heat-resistant plate, propagation of vibrationcaused in the cBN sintered body during working to the tool shank portioncan be suppressed. Namely, by providing the vibration-isolatingheat-resistant plate, load caused by vibration on the tool shank portionduring working can be lessened.

Preferably, the vibration-isolating heat-resistant plate has thermalconductivity not higher than 40 W/m·K. As the vibration-isolatingheat-resistant plate exhibits thermal conductivity not higher than 40W/m·K, friction heat generated during working is less likely to conductto the tool shank portion but it can conduct to a workpiece. Softeningof the workpiece can thus be promoted and hence chipping of the toolmade of the cBN sintered body can be less likely. Such avibration-isolating heat-resistant plate has thermal conductivity morepreferably not higher than 20 W/m·K and further preferably not higherthan 5 W/m·K. In addition, by using a vibration-isolating heat-resistantplate made of an oxide, the vibration-isolating heat-resistant plate canhave further lower thermal conductivity. Moreover, a vibration-isolatingheat-resistant plate preferably has a thickness not smaller than 0.3 mm.Thus, such an effect that heat radiation to the tool shank portion issuppressed and strength sufficient to withstand cutting is obtained canbe achieved.

<Method of Manufacturing cBN Sintered Body>

The cBN sintered body employed in the present invention can be obtainedby introducing cBN particles, source material powders forming the heatinsulating phase, and source material powders forming the binder phasein an ultra-high pressure apparatus and then subjecting these powders toultra-high pressure sintering. By thus including the source materialpowders forming the heat insulating phase and then carrying outultra-high pressure sintering, thermal conductivity of the cBN sinteredbody can be lowered. Here, as a condition for ultra-high pressuresintering, a pressure during ultra-high pressure sintering is preferablylow, and more specifically, the pressure is preferably not lower than 2GPa and not higher than 7 GPa. A temperature during ultra-high pressuresintering is preferably not lower than 1100° C. and not higher than1800° C. and a time period required for ultra-high pressure sinteringtreatment is preferably not shorter than 5 minutes and not longer than30 minutes.

Further, low-pressure sintering may be carried out as a sintering methodother than ultra-high pressure sintering above. Then, sintering ofsource material powders forming the heat insulating phase is less likelyto proceed completely, unsintered regions can intentionally be scatteredas a part of the heat insulating phase, and an effect to prevent heatconduction can be obtained. Here, as low-pressure sintering, forexample, a hot pressing method or a spark plasma sintering method can beapplied.

EXAMPLES

Though the present invention will be described further in detail withreference to examples, the present invention is not limited thereto.

Example 1

A tool made of a cBN sintered body was fabricated as below. Initially, acompound obtained by mixing WC powders having an average particle sizeof 1.3 μm, Co powders having an average particle size of 1.1 μm, and Alpowders having an average particle size of 4 μm at a mass ratio ofWC:Co:Al=25:68:7 and then subjecting the mixture to heat treatment undervacuum at 1000° C. for 30 minutes was crushed with a ball of φ4 mm madeof cemented carbide, to thereby obtain source material powders formingthe binder phase.

Then, as a component for the first compound forming the heat insulatingphase, a mixture of Al powders having an average particle size of 0.85μm and Zr powders having an average particle size of 0.7 μm wassubjected to heat treatment in a nitrogen atmosphere at 1000° C. for 30minutes to thereby fabricate a compound. Thereafter, the compound wascoarsely crushed, and then a medium having a diameter of φ0.6 mm andmade of zirconia was employed, and the medium and the compound werefinely crushed in an ethanol solvent at a flow rate of 0.2 L/min. Themedium used for crushing was then removed and the source materialpowders of the first compound forming the heat insulating phase wereprepared.

Then, the source material powders forming the binder phase, the sourcematerial powders of the first compound forming the heat insulatingphase, and the cBN powders having an average particle size of 0.9 μmobtained as above were blended, mixed, and dried such that a cBN contentafter sintering attained to 60 volume %. Further, these powders werelayered on a support plate made of cemented carbide and loaded into acapsule made of Mo. Thereafter, the powders were sintered in anultra-high pressure apparatus at a pressure of 7 GPa at a temperature of1750° C. for 30 minutes, to thereby obtain the cBN sintered body havingcomposition and thermal conductivity shown in Table 1 below. Inaddition, composition of the compound forming the binder phase was foundby using X-ray diffraction and shown in the field of “binder phase” inTable 1.

The cBN sintered body obtained as above was cut in a prescribed shapeand fixed to a tool shank portion with a vibration-isolatingheat-resistant plate being interposed, to thereby fabricate a tool madeof a cBN sintered body. The tool made of the cBN sintered body thusfabricated was ground to a prescribed tool shape. Here, a tool shankportion made of cemented carbide was employed as the tool shank portion,and a vibration-isolating heat-resistant plate composed of an oxide ofZr, having a thickness not smaller than 1 mm, and having thermalconductivity of 3 W/m·K was employed.

Surface roughness Rz at the tool working point of the tool made of thecBN sintered body thus fabricated was measured with a surface roughnessmeasuring instrument (SURFCOM 2800E (manufactured by Tokyo Seimitsu Co.,Ltd.)). Rz at the tool working point of the tool made of the cBNsintered body was 2.3 μm.

Examples 2 to 3

Tools made of cBN sintered bodies according to Examples 2 to 3respectively were fabricated with the method the same as in Example 1except that a cBN content was different as in Table 1 from the tool madeof the cBN sintered body according to Example 1.

Examples 4 to 6

Tools made of cBN sintered bodies according to Examples 4 to 6respectively were fabricated with the method the same as in Example 1except that a cBN content and composition in the heat insulating phasewere different as in Table 1 from the tool made of the cBN sintered bodyaccording to Example 1.

For example, in Example 4, as a component for the first compound formingthe heat insulating phase, a mixture of Ti powders having an averageparticle size of 0.9 μm and Zr powders having an average particle sizeof 0.7 μm was employed. Similarly, in Example 5, as a component for thefirst compound in the heat insulating phase, a mixture of Ti powdershaving an average particle size of 0.9 μm and Si powders having anaverage particle size of 0.8 μm was employed. In Example 6, as thecomponent for forming the heat insulating phase, Al powders having anaverage particle size of 0.85 μm and Zr powders having an averageparticle size of 0.7 μm was employed for the component for the firstcompound, and ammonium paratungstate (5(NH₄)₂O.12WO₃.5H₂O) powdershaving an average particle size of 0.6 μm and ammonium perrhenate(NH₄ReO₄) powders having an average particle size of 0.8 μm were used asthe source material powders for the second compound.

Examples 7 to 8

Tools made of cBN sintered bodies according to Examples 7 to 8respectively were fabricated with the method the same as in Example 1except that an average particle size of the first compound forming theheat insulating phase was different as in Table 1 from the tool made ofthe cBN sintered body according to Example 1.

For example, in Example 7, a medium having a diameter of φ0.3 mm andmade of zirconia was used to fabricate source material powders of thefirst compound, and these source material powders were used to fabricatea tool made of a cBN sintered body containing the first compound havingan average particle size of 30 nm. Here, an average particle size of thefirst compound was obtained by lapping the cBN sintered body to a mirrorsurface, magnifying the cBN sintered body with an electron microscope to×50000, measuring a particle size of the first compound in the heatinsulating phase at 10 sites, and calculating an average value.

Further, in Example 8, a medium having a diameter of φ1.0 mm and made ofzirconia was used to fabricate source material powders of the firstcompound, and these source material powders were used to fabricate atool made of a cBN sintered body containing the first compound having anaverage particle size of 95 nm.

Examples 9 to 10

Tools made of cBN sintered bodies were fabricated with the method thesame as in Example 4 except that volume % of an unsintered region wasdifferent as in Table 1 from the tool made of the cBN sintered bodyaccording to Example 4, by differing a pressure for sintering. Forexample, in Example 9, by sintering the cBN powders, the source materialpowders of the first compound forming the heat insulating phase, and thesource material powders forming the binder phase with a pressure duringsintering being set to 5.5 GPa, the tool made of the cBN sintered bodyincluding an unsintered region by 0.01% with respect to the cBN sinteredbody was obtained. In addition, in Example 10, by sintering the cBNpowders, the source material powders of the first compound forming theheat insulating phase, and the source material powders forming thebinder phase with a pressure during sintering being set to 2.5 GPa, thetool made of the cBN sintered body including an unsintered region by0.5% with respect to the cBN sintered body was obtained.

Example 11

A tool made of a cBN sintered body was fabricated by using a sparkplasma sintering (SPS) apparatus instead of an ultra-high pressuresintering apparatus, as compared with the tool made of the cBN sinteredbody according to Example 4. Specifically, by sintering the cBN powders,the source material powders forming the binder phase, and the sourcematerial powders of the first compound forming the heat insulating phasewith a temperature in the SPS apparatus being set to 1500° C. and apressure during sintering being adjusted to 0.05 GPa, the cBN sinteredbody was obtained. A method of fabricating a cBN sintered body with theuse of the SPS apparatus will specifically be described. A mixture ofcBN powders, source material powders forming the binder phase, andsource material powders of the first compound forming the heatinsulating phase was loaded into a mold for sintering made of graphite,a pressure was increased to 0.05 GPa, a temperature in the apparatus wasset to 1500° C. under a vacuum heating condition, and spark plasmasintering was carried out for 30 minutes or shorter (see, for example,paragraph [0014] of Japanese Patent Laying-Open No. 2008-121046).

The cBN sintered body thus obtained was cut across one plane, and thecross-section was observed and analyzed at ×10000 by using a TEM.Consequently, it was confirmed that 1.5% of the cross-sectional area ofthe cross-section was unsintered. Thus, it was clarified that the cBNsintered body included an unsintered region by 1.5 volume %.

In addition, it was confirmed that a partial region was hexagonal, as aresult of X-ray diffraction of the cBN sintered body obtained in thepresent Example. It was thus clarified that the cubic boron nitridesintered body according to Example 11 partially included hexagonal boronnitride (hBN). Such generation of hBN is estimated to probably haveresulted from inverse transformation from cBN to hBN due to a lowsintering pressure during sintering.

Example 12

A tool made of a cBN sintered body was fabricated by using a hotpressing apparatus instead of an ultra-high pressure sinteringapparatus, as compared with the tool made of the cBN sintered bodyaccording to Example 4. Specifically, by sintering the cBN powders, thesource material powders forming the binder phase, and the sourcematerial powders of the first compound forming the heat insulating phasewith a temperature in the hot pressing apparatus being set to 1500° C.and a pressure during sintering being adjusted to 0.03 GPa, the cBNsintered body was obtained.

The cBN sintered body thus obtained was cut across one plane, and thecross-section was observed and analyzed at ×10000 by using a TEM. Then,it was confirmed that 3% of the cross-sectional area of thecross-section was unsintered. Thus, it was clarified that the cBNsintered body included an unsintered region by 3 volume %.

In addition, it was confirmed that hexagonal boron nitride (hBN) waspartially included as in Example 11, as a result of X-ray diffraction ofthe cBN sintered body obtained in the present Example.

The tool made of the cBN sintered body according to each Example thusfabricated is a tool made of a cubic boron nitride sintered body whichincludes a cubic boron nitride sintered body at least at a tool workingpoint, the cubic boron nitride sintered body contains cubic boronnitride, a heat insulating phase, and a binder phase, cubic boronnitride is contained in the cubic boron nitride sintered body by notless than 60 volume % and not more than 99 volume %, the heat insulatingphase contains one or more types of first compound composed of one ormore types of element selected from the group consisting of Al, Si, Ti,and Zr and one or more types of element selected from the groupconsisting of N, C, O, and B by not less than 1 mass % and not more than20 mass %, and the cubic boron nitride sintered body has thermalconductivity not higher than 70 W/m·K.

Comparative Examples 1 to 2

Tools made of cubic boron nitride sintered bodies according toComparative Examples 1 to 2 respectively were fabricated with the methodthe same as in Example 1 except that a cBN content and composition inthe binder phase were different as in Table 1 from the tool made of thecubic boron nitride sintered body according to Example 1 and that theheat insulating phase was not included. An average particle size of acomponent forming the binder phase in the cubic boron nitride sinteredbody thus fabricated was measured and the average particle size was 100nm or greater in each case.

Comparative Example 3

A tool made of a cubic boron nitride sintered body according toComparative Example 3 was fabricated with the method the same as inExample 1 except that a cBN content after sintering was set to 80 volume% and source material powders of the first compound having an averageparticle size of 200 nm were fabricated and contained by using a mediumhaving a diameter of φ3.5 mm and made of cemented carbide, as comparedwith the tool made of the cubic boron nitride sintered body according toExample 1.

TABLE 1 Unsintered Pressure Thermal cBN Average Particle Region DuringConduc- Content Heat Insulating Phase Size of First (Volume Sinteringtivity (Volume %) Binder Phase (Mass %) Compound (nm) %) (GPa) (W/m · K)Example 1 60 WC, W₂Co₂₁B₆, Co₃W₃C, AlB₂ Al₂O₃ (3%), ZrC (2%) 45 — 7 35Example 2 75 WC, W₂Co₂₁B₆, Co₃W₃C, AlB₂ Al₂O₃ (3%), ZrC (2%) 45 — 7 48Example 3 80 WC, W₂Co₂₁B₆, Co₃W₃C, AlB₂ Al₂O₃ (3%), ZrC (2%) 45 — 7 55Example 4 85 WC, W₂Co₂₁B₆, Co₃W₃C, AlB₂ TiC (1.7%), ZrO₂ (1.2%) 45 — 763 Example 5 90 WC, W₂Co₂₁B₆, Co₃W₃C, AlB₂ TiCN (1.2%), Si₃N₄ (1.1%) 45— 7 67 Example 6 99 WC, W₂Co₂₁B₆, Co₃W₃C, AlB₂ Al₂O₃ (0.3%), ZrO₂ (0.4%)45 — 7 69 WO₃ (0.1%), ReO₄ (0.2%) Example 7 60 WC, W₂Co₂₁B₆, Co₃W₃C,AlB₂ Al₂O₃ (3%), ZrC (2%) 30 — 7 30 Example 8 60 WC, W₂Co₂₁B₆, Co₃W₃C,AlB₂ Al₂O₃ (3%), ZrC (2%) 95 — 7 45 Example 9 85 WC, W₂Co₂₁B₆, Co₃W₃C,AlB₂ TiC (1.7%), ZrO₂ (1.2%) 45  0.01 5.5 55 Example 10 85 WC, W₂Co₂₁B₆,Co₃W₃C, AlB₂ TiC (1.7%), ZrO₂ (1.2%) 45 0.5 2.5 42 Example 11 85 WC,W₂Co₂₁B₆, Co₃W₃C, AlB₂ TiC (1.7%), Zr0₂ (1.2%) 45 1.5 0.05*¹ 35 Example12 85 WC, W₂Co₂₁B₆, Co₃W₃C, AlB₂ TiC (1.7%), ZrO₂ (1.2%) 45 3   0.03*²27 Comparative 50 TiN, TiB₂, AlN None — — 7 40 Example 1 Comparative 85WC, CoWB, AlB₂ None — — 7 90 Example 2 Comparative 80 WC, W₂Co₂₁B₆,Co₃W₃C, AlB₂ Al₂O₃ (3%), ZrC (2%) 200 — 7 80 Example 3 *¹The cBNsintered body was fabricated with the use of the spark plasma sinteringapparatus. *²The cBN sintered body was fabricated with the use of thehot pressing apparatus.

Here, “cBN content” in Table 1 was calculated as follows. Initially, thecBN sintered body fabricated in each Example and each ComparativeExample was mirror-polished (a thickness to be polished being smallerthan 50 μm), and a cBN sintered body structure in an arbitrary regionwas photographed at ×10000 with an electron microscope. Then, a blackregion, a gray region, and a white region were observed. With anattached EDX, it was confirmed that the black region represented cBNparticles, and the gray region and the white region represented thebinder phase. Further, it was also confirmed that the gray regionrepresented a Co compound, a Ti compound, and an Al compound, and thewhite region represented a W compound.

Then, the photograph at ×10000 taken as above was subjected tobinarization processing by using image processing software and a totalarea of the regions occupied by the cBN particles (the black regions) inthe photograph was calculated. A percentage of the ratio of the blackregions occupied in the cBN sintered body in the photograph was definedas “cBN content” in Table 1 expressed in volume %.

In addition, “thermal conductivity” in Table 1 was calculated based onthermal diffusivity of the cBN sintered body obtained by measurementwith a laser flash method and on specific heat and density of the cBNsintered body calculated with a different method.

The cubic boron nitride sintered body according to each Example and eachComparative Example thus obtained was used to fabricate a tool made ofthe cBN sintered body having the following tool shape. Then, the toolmade of the cBN sintered body was subjected to cutting tests 1 and 2 andplasticity tests 1 and 2. Tables 2 to 5 show the results.

Cutting Test 1

The tools made of the cBN sintered bodies, of a tool model numberSNMA120430, were fabricated in accordance with Examples 1 to 6 andComparative Examples 1 to 3 and they were subjected to a cutting testunder the following conditions.

Work material: Working of outer diameter of Ni-basedultra-heat-resistant alloy Inconel 718

Hardness of work material: Hv 430

Cutting condition: Cutting speed V=200 m/min.

-   -   Amount of feed f=0.15 mm/rev.    -   Cutting depth d=0.15 mm    -   Coolant Emulsion of 20-fold dilution

TABLE 2 Distance of Cutting Until Tool Life Was Reached (km) Form ofDamage Example 1 1.4 Boundary Chipped Example 2 1.7 Boundary ChippedExample 3 2.1 Normal Wear Example 4 1.95 Normal Wear Example 5 1.8Normal Wear Example 6 1.7 Normal Wear Comparative 0.4 Boundary ChippedExample 1 Comparative 0.6 Boundary Chipped Example 2 Comparative 0.7Boundary Chipped Example 3

“Distance of cutting until tool life was reached” in Table 2 representsa distance of cutting (km) at the time point when a wear width of thecBN sintered body exceeded 0.3 mm in a case where no chipping was causedbefore the wear width exceeds 0.3 mm, and it represents a distance ofcutting (km) until chipping was caused in a case where chipping wascaused before the wear width exceeds 0.3 mm, with the cutting test beingstopped at that time point. It is noted that a longer distance ofcutting indicates a longer tool life.

In addition, “form of damage” in Table 2 shows “normal wear” when a wearwidth of the cBN sintered body after the cutting test exceeded 0.3 mmand shows “boundary chipped” in a case where chipping was caused beforethat.

As can clearly be seen in Table 2, it is evident that the tools made ofthe cubic boron nitride sintered bodies according to the presentinvention in Examples 1 to 6 have a longer tool life than the tools madeof the cubic boron nitride sintered bodies in Comparative Examples 1 to3 respectively.

Among Examples 1 to 6, the tool made of the cubic boron nitride sinteredbody according to Example 3 is considered to have the longest lifebecause thermal conductivity of the cBN sintered body is not higher than60 W/m·K and a cBN content therein is 80 volume %. In contrast, the toolmade of the cubic boron nitride sintered body according to ComparativeExample 1 is considered to have a short tool life because a cBN contentis as low as 50 volume % and hence strength is low, although thermalconductivity of the cBN sintered body is not higher than 60 W·K and ithas relatively low thermal conductivity.

Further, though the tool made of the cubic boron nitride sintered bodyaccording to Comparative Example 2 has a cBN content of 85 volume %, ithas relatively high thermal conductivity of the cBN sintered body of 90W/m·K, because it does not include the heat insulating phase. Therefore,it is estimated that heat generated during cutting was less likely toconduct to a work material and the work material could not sufficientlybe softened, which led to boundary chipping in an early stage.Furthermore, the tool made of the cubic boron nitride sintered bodyaccording to Comparative Example 3 has an average particle size of thefirst compound not smaller than 100 μm and hence an effect of the heatinsulating phase cannot be obtained. Thus, thermal conductivity of thecBN sintered body is relatively high, that is, around 80 W/m·K.Therefore, it is estimated that heat generated during cutting was lesslikely to conduct to a work material and the work material could notsufficiently be softened, which led to boundary chipping in an earlystage.

Cutting Test 2

In Examples 9 to 12 and Comparative Example 2, the tools made of the cBNsintered bodies, of a tool model number CNGA120408, were fabricated andsubjected to a cutting test under the following conditions.

Work material: 0.8C-2.0Cu-remainder Fe (JPMA notation: SMF4040)

Work material hardness: 78 HRB

Cutting condition: Cutting speed V=200 m/min.

-   -   Amount of feed f=0.1 mm/rev.    -   Cutting depth ap=0.2 mm    -   Cutting fluid Used

TABLE 3 Distance of Cutting Until Tool Life Was Reached (km) Form ofDamage Example 9 9.1 Normal Wear Example 10 10.6 Normal Wear Example 117.8 Normal Wear Example 12 6.1 Small Chipping Comparative 0.1 NormalWear Example 2

“Form of damage” in Table 3 shows “small chipping” when chipping to suchan extent as visually observed in a surface of the cBN sintered bodyafter the cutting test was caused. It is noted that other forms ofdamage were determined based on the criteria as in cutting test 1.

As can clearly be seen in Table 3, it is evident that the tools made ofthe cubic boron nitride sintered bodies according to the presentinvention in Examples 9 to 12 have longer tool life than the tool madeof the cubic boron nitride sintered body according to ComparativeExample 2.

The reason why the tool life of the tool made of the cubic boron nitridesintered body according to Comparative Example 2 was short may bebecause thermal conductivity of the cubic boron nitride sintered bodywas higher than 70 W/m·K, a relatively large amount of heat generated byworking flowed into the tool, softening of a work material wasconsequently not promoted sufficiently, shear of the work material atthe tool working point did not smoothly proceed, pits were caused in aworked surface from an initial stage of working, and surface roughnessof the worked surface became poor.

Plasticity Test 1 Punch Pressing

In Examples 1, 7, and 8 and Comparative Examples 1 to 3, the tools madeof the cBN sintered bodies having a cylindrical tool shape of φ 10 werefabricated and subjected to a plasticity test under the followingconditions.

Workpiece: SUS304

Hardness of workpiece: Hv 180

Thickness of workpiece: 2 mm

Plasticity Condition: Punch-pressing load of 2.5 GPa

TABLE 4 The Number of Times of Punching (Times) Example 1 23500 Example7 25000 Example 8 20000 Comparative 5000 Example 1 Comparative 6000Example 2 Comparative 8000 Example 3

“The number of times of punching” in Table 4 shows the number of timesof punching the workpiece before creation of burr in a punched hole. Itis noted that the greater number of times of punching indicatesimprovement in hardness of a tool made of a cubic boron nitride sinteredbody and a longer tool life.

As can clearly be seen in Table 4, it is evident that the tools made ofthe cubic boron nitride sintered bodies according to the presentinvention in Examples 1, 7, and 8 have a longer tool life than the toolsmade of the cubic boron nitride sintered bodies in Comparative Examples1 to 3. Thus, it was confirmed that a life of a tool made of a cubicboron nitride sintered body was improved.

Plasticity Test 2 Friction Compression Joint

In Examples 1, 7, and 8 and Comparative Examples 1 to 3, a special toolin which a vibration-isolating heat-resistant plate having a thicknessof 2 mm and made of zirconia was brazed to a bottom surface of the toolmade of the cBN sintered body where a protrusion in an M4 left-handscrew shape having a screw height of 3 mm was formed in a centralportion of a column having a diameter of 12.7 mm was fabricated andsubjected to a plasticity test under the following conditions.

Material to be joined: Two-layered high-tensile steel

Tensile strength of material to be joined: 590 MPa

Thickness of material to be joined: 1 mm

Joint conditions: The number of revolutions of 2500 rpm

-   -   Pressurizing force of 10000 N

TABLE 5 The Number of Times of Joint (Times) Example 1 11000 Example 712000 Example 8 7000 Comparative 200 Example 1 Comparative 100 Example 2Comparative 150 Example 3

“The number of times of joint” in Table 5 shows the number of times ofjoining the material to be joined before a screw portion of the toolmade of the cBN sintered body was chipped. It is noted that the greaternumber of times of joint indicates a longer tool life.

As can clearly be seen in Table 5, it is evident that the tools made ofthe cubic boron nitride sintered bodies according to the presentinvention in Examples 1, 7, and 8 have a longer tool life than the toolsmade of the cubic boron nitride sintered bodies in Comparative Examples1 to 3.

Though the embodiments and the examples of the present invention havebeen described as above, combination of the features in the embodimentsand the examples described above as appropriate is also originallyintended.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the scopeof the present invention being interpreted by the terms of the appendedclaims.

What is claimed is:
 1. A tool made of a cubic boron nitride sinteredbody, comprising a cubic boron nitride sintered body at least at a toolworking point, said cubic boron nitride sintered body containing cubicboron nitride, a heat insulating phase, and a binder phase, said cubicboron nitride being contained in said cubic boron nitride sintered bodyby not less than 60 volume % and not more than 99 volume %, said heatinsulating phase containing one or more types of first compound composedof one or more types of element selected from the group consisting ofAl, Si, Ti, and Zr and one or more types of element selected from thegroup consisting of N, C, O, and B, said first compound being containedin said cubic boron nitride sintered body by not less than 1 mass % andnot more than 20 mass % and having an average particle size smaller than100 nm, and said cubic boron nitride sintered body having thermalconductivity not higher than 70 W/m·K.
 2. The tool made of a cubic boronnitride sintered body according to claim 1, wherein said first compoundhas an average particle size smaller than 50 nm.
 3. The tool made of acubic boron nitride sintered body according to claim 1, wherein saidheat insulating phase contains as its part, an unsintered region by notless than 0.01 volume % and not more than 3 volume % with respect tosaid cubic boron nitride sintered body.
 4. The tool made of a cubicboron nitride sintered body according to claim 1, wherein said firstcompound is a compound in which a solid solution of any one or both ofoxygen and boron is present by not less than 0.1 mass % and not morethan 10 mass % with respect to a nitride, a carbide, and a carbonitrideof one or more types of element selected from the group consisting ofAl, Si, Ti, and Zr.
 5. The tool made of a cubic boron nitride sinteredbody according to claim 1, wherein said heat insulating phase containsone or more types of second compound composed of W and/or Re and one ormore types of element selected from the group consisting of N, C, O, andB, in addition to said first compound, and said second compound iscontained in said cubic boron nitride sintered body by not less than 0.1mass % and not more than 2 mass %.
 6. The tool made of a cubic boronnitride sintered body according to claim 1, wherein said cubic boronnitride is contained in said cubic boron nitride sintered body by notless than 75 volume % and not more than 92 volume %.
 7. The tool made ofa cubic boron nitride sintered body according to claim 1, wherein saidcubic boron nitride is contained in said cubic boron nitride sinteredbody by not less than 80 volume % and not more than 87 volume %.
 8. Thetool made of a cubic boron nitride sintered body according to claim 1,wherein said cubic boron nitride is composed of cubic boron nitrideparticles having an average particle size not greater than 1 μm.
 9. Thetool made of a cubic boron nitride sintered body according to claim 1,wherein said cubic boron nitride sintered body has thermal conductivitynot higher than 60 W/m·K.
 10. The tool made of a cubic boron nitridesintered body according to claim 1, wherein said tool working point hassurface roughness Rz not less than 1 μm and not more than 20 μm.
 11. Thetool made of a cubic boron nitride sintered body according to claim 1,wherein said cubic boron nitride sintered body has a minimum thicknessnot smaller than 2 mm at said tool working point.
 12. The tool made of acubic boron nitride sintered body according to claim 1, wherein saidcubic boron nitride sintered body and a tool shank portion are fixed toeach other with a vibration-isolating heat-resistant plate beinginterposed, and said vibration-isolating heat-resistant plate is made ofan oxide, has thermal conductivity not higher than 40 W/m·K, and has athickness not smaller than 0.3 mm.
 13. The tool made of a cubic boronnitride sintered body according to claim 12, wherein said cubic boronnitride sintered body and said tool shank portion are fixed to eachother by screwing and/or self-gripping.